1. Field of the Invention
This invention relates generally to multiple-input, multiple-output wireless local area networks, and more particularly to selecting antennas and beams in such networks.
2. Description of the Related Art
Multiple-input, multiple-output (MIMO) techniques can significantly increase system capacity in a scattering environment of a wireless network. However, the use of more antennas increases the hardware complexity and cost because in a typical system, each transmit/receive antenna requires a separate RF chain including a modulator/demodulator, an AD/DA converter, an up/down converter, and a power amplifier. In addition, the processing complexity at the baseband also increases with the number of antennas.
Antenna/beam selection can reduce the number of RF chains while still taking advantage of the capacity/diversity increase provided by multiple antennas. In a wireless local area network (WLAN), stations are typically operated at a high signal to noise ratio (SNR), in which diversity plays a key role in protecting the system from a deep fading channel. Furthermore, it is known that the state of a WLAN channel changes slowly. Therefore, it is advantageous to perform antenna/beam selection in a WLAN.
The idea of antenna/beam selection is to select a submatrix from a complete channel matrix or a transformed channel matrix for beam selection, according to some predetermined criteria. To perform antenna/beam selection, the complete channel matrix is estimated by sending training (sounding) frames that enable the antenna selection station to measure the complete channel state information (CSI). Conventionally, explicit signaling is used in the physical (PHY) or media access (MAC) layer by sending training frame(s) for all the antennas to be selected. However, the additional overheads are undesirable due to practical limitations. On the other hand, the slowly varying WLAN channel environment can advocate a more efficient antenna/beam selection training scheme which requires little or no changes in the MAC and PHY layers.
Structure of IEEE 802.11n WLAN Link Adaptation Control (LAC) Mechanisms in MAC Layer
As shown in FIG. 1 and FIG. 12, the WLAN IEEE 802.11n standard, incorporated herein by reference, also known as WiFi, proposes to specify a fast link adaptation control (LAC) mechanism defined at the MAC layer for supporting MIMO training requests and exchange of link adaptation information. In general, the LAC functionality can be realized either by a single control frame defined as LAC, or the single control frame can be a High Throughput (HT) Control frame, or a HT Control (HTC) Field can be incorporated into any MAC layer frame, which is named as +HTC frame. As shown in FIG. 1, LAC frame contains the following fields: a MAC header 110, a LAC mask 120 for indicating the logical elements carried in the current control frame, a modulation coding scheme (MCS) feedback field 130 for indicating transmitting parameters, and a frame check sequence (FCS) 140 for error detection. The MAC header 110 applies for any MAC layer packet, which includes a frame control 111, duration 112, a receive address (RA) 113, and a transmit address (TA) 114. The LAC frame is described in detail in IEEE 802.11-04/0889r7, “TGn Sync Proposal Technical Specification,” incorporated herein by reference.
The LAC frame supports control of MIMO training requests and exchange of link adaptation information. The LAC frame can be sent by either an initiator station (transmitter) or a recipient station (receiver).
FIG. 2 shows the LAC mask field 120 in greater detail. Without considering antenna/beam selection, the LAC mask field 120 includes the following: RTS (request to send) 121, CTS (clear to send) 122, TRQ (MIMO training request) 123, MRQ (request for MCS feedback) 124, and MFB (MCS feedback) 125. The three bits 126 are reserved. In the MCS feedback case, i.e., MFB=1, the MCS set is indicated in the ‘MCS feedback’ field 130 in FIG. 1.
An HT Control Field 1200 includes a LAC field 1201 which controls the fast link adaptation training process; and several other fields 1202 dedicated for other HT control features. Without considering antenna/beam selection, the LAC field of an HT control field includes: MA 1210, TRQ 1220, MRQ 1230, MRS 1240 (MRQ sequence number), MFS 1250 (MFB sequence number), and MFB 1260 with 7 bits functioning as the MCS feedback field in the above LAC frame. The HT Control Field is described in detail in IEEE 802.11-05/1095r3, “Joint Proposal: High throughput extension to the 802.11 Standard: MAC,” incorporated herein by reference.
Structure of IEEE 802.11n WLAN Channel Sounding Mechanisms Defined in PHY Layer
A sounding packet is defined as any packet containing the training information (residing in PHY layer header) of all the available transmitting chains (or MIMO channel dimensions). There are two major categories of sounding packets defined in the PHY layer of high throughput WLAN: the first one is regular sounding packets, which can be any ordinary packet with the additional training information for the extra channel dimensions other than those used for data transmissions, if there is any; the second category is named as zero-length frame (ZLF), which contains only PHY layer header with the training information of all the available transmitting chains. Based on the above definitions, a regular sounding packet may contain a HT control field in the MAC header (i.e. a +HTC frame), while ZLF is not allowed to contain HT control field. Therefore any MAC layer signaling in a sounding packet (e.g., in TxBF or antenna selection) with ZLF format should be designed in a way different from that with regular sounding packet. Note that the regular +HTC frame sent immediate before one ZLF or several consecutive ZLFs should indicate the subsequent ZLF(s) in its HT control field (by setting the ZLF bit as in FIG. 12), and the subsequent ZLF(s) shall follow the same destination address as that of the immediate previous +HTC frame. The sounding packets is described in detail in IEEE 802.11-05/1102r2, “Joint Proposal: High throughput extension to the 802.11 Standard: PHY,” incorporated herein by reference.
Closed-Loop MIMO Training Methods for IEEE 802.11n WLAN
The IEEE 802.11n standard requires a throughput of 100 megabits per second (Mbps) at the medium access control (MAC) layer service access point (SAP). Based on the channel property in WLAN environment, closed-loop schemes are preferred for increased throughput, including transmit beam forming (TXBF), MCS adaptation, and antenna/beam selection.
Each PHY layer packet is composed by two portions: preamble and data. The PHY packet preamble includes training information for channel estimation at the receiver. Typically, in a conventional PHY layer packet, the number of antennas or spatial streams indicated in the training field can be less than the maximum number provided by the MIMO channel. A sounding packet is a specific PHY layer packet, which contains the training information for all the available data streams in the MIMO channel, no matter how many data streams are used for transmitting the data portion. When the concept of sounding packet is not applied in the system, an alternative category of PHY layer training packet is the one that enforces a MCS set utilizing all the available data streams in the MIMO channel, so that not only the preamble contains the full training information of the MIMO channel, the data portion is also transmitted using all the available data streams.
MCS Training Process
FIG. 3 shows a conventional MIMO training process for MCS adaptation based on LAC frame however, it should be understood that an HT control field can also be used. An initiator (transmit) station STA A 301 sends a LAC frame 310 with MRQ=1, or a frame containing HT Control Field with MRQ=1, and MRS equal to a corresponding sequence number, to a recipient (receive) station STA B 302. The initiator also requests its PHY layer to signal a sounding packet. In response to receiving the MRQ and the sounding packet, the recipient 302 estimates the MIMO channel and determines an appropriate MCS set for the current channel. Then, the recipient replies to the initiator a LAC frame 320 with MFB set to 1, and the MCS feedback field 130 contains the selected MCS set, or a frame including a HT Control Field with MFS equal to the MRS in the received frame it is currently responding to, and with MFB including a selected MCS set.
The recipient 302 can also initiate the MCS training process whenever it has the complete MIMO channel knowledge, by determining the MCS and sending an MFB with MCS feedback directly without any matching MRQ element. This is called unsolicited adaptation.
TXBF Training Process
FIG. 4 shows a conventional transmit beam forming (TXBF) training process based on LAC frame. Again, it should be understood that a HT control field can be used, if corresponding TXBF training functionalities are defined in the reserved fields described above. The initiator 301 sends out a LAC frame 410 with TRQ set to 1 to the recipient 302. In response to receiving the TRQ, the recipient sends back a sounding packet 420 to the initiator. Upon receiving the sounding packet, the initiator estimates the MIMO channel and updates its beam forming steering matrices. Up to now, recipient initiated TXBF training is not defined.
For antenna selection, some prior art training methods use a single PHY layer training frame (e.g., sounding packet) containing the training information for all the antennas to be selected, and different antenna subsets are subsequently connected to the RF chains for this single training frame. This introduces overhead on existing training frame designs.
In another training method, a long sequence of training frames is transmitted from a receive station to a transmit station, and in response the transmit station transmits a short sequence of training frames so that both the transmit and receive station can perform channel estimation and antenna selection, see U.S. patent application Ser. No. 11/127,006 “Training Frames for MIMO Stations,” filed by Andreas Molisch, Jianxuan Du and Daqing Gu on May 11, 2005, incorporated herein by reference.